When Charles succeeded his brother in 1711, he was the last male Habsburg heir in the direct line. Since Habsburg possessions were subject to Salic law, barring women from inheriting in their own right, his own lack of a male heir meant they would be divided on his death. The Pragmatic Sanction of 19 April 1713 abolished male-only succession in all Habsburg realms and declared their lands indivisible, although Hungary only approved it in 1723.[10]

Charles had three daughters, Maria Theresa (1717-1780), Maria Anna (1718-1744) and Maria Amalia (1724-1730) but no suriving sons. When Maria Theresa was born, he disinherited his nieces and the daughters of his elder brother Joseph, Maria Josepha and Maria Amalia. It was this act that undermined the chances of a smooth succession and obliged Charles to spend the rest of his reign seeking to ensure enforcement of the Sanction from other European powers.[11]

They exacted harsh terms; in 1731, Charles agreed to a demand from Britain that he close a trade competitor, the Ostend Company, which was based in the Austrian Netherlands and that he himself founded in 1722.[12] However, by 1735 he had secured approvals from key states, most importantly the Imperial Diet, which in theory bound all its members including Prussia and Bavaria.

Other signatories included Britain, France, the Dutch Republic, Spain, Russia, Denmark and Savoy-Sardinia but subsequent events underlined Eugene of Savoy's comment that the best guarantee was a powerful army and full Treasury. His nieces were married to the rulers of Saxony and Bavaria, both of whom ultimately refused to be bound by the decision of the Imperial Diet and despite publicly agreeing to the Pragmatic Sanction in 1735, France signed a secret treaty with Bavaria in 1738 promising to back the 'just claims' of Charles Albert of Bavaria.[13]

Charles VI with his wife Empress Elisabeth Christine and their daughters in 1730

The issue of Charles' elder daughter's marriage was raised early in her childhood. She was first betrothed to Léopold Clément of Lorraine, who was supposed to come to Vienna and meet Maria Theresa. Instead, he died of smallpox in 1723, which upset Maria Theresa. Léopold Clément's younger brother, Francis Stephen, then came to Vienna to replace him. Charles considered other possibilities (such as Don Carlos) before announcing the engagement to Francis.[17] At the end of the War of the Polish Succession, France demanded that Francis surrender the Duchy of Lorraine (his hereditary domain), to Stanisław Leszczyński, the deposed King of Poland, who would bequeath it to France at his death. Charles compelled Francis to renounce his rights to Lorraine and told him: "No renunciation, no archduchess."[18] Francis complied; he was married to Maria Theresa in February 1736, and Lorraine devolved to Stanisław in July 1737.

In 1737, the Emperor embarked on another Turkish War in alliance with Russia. Unlike the previous Austro-Turkish War, it ended in a decisive Austrian defeat. Much of the territory gained in 1718 (Except for the Banat) was lost. Popular discontent at the costly war reigned in Vienna; Francis of Lorraine, Maria Theresa's husband, was dubbed a French spy by the Viennese.[19] The war not only revealed the sorry state of the Austrian army, but also the weakness of the imperial state itself, which did not have the financial strength to sustain a long war without the subsidies of its allies.

At the time of Charles' death, the Habsburg lands were saturated in debt; the exchequer contained a mere 100,000 florins; and desertion was rife in Austria's sporadic army, spread across the Empire in small, ineffective barracks.[20] Contemporaries expected that Austria-Hungary would wrench itself from the Habsburg yoke upon his death.[20]

The Emperor, after a hunting trip across the Hungarian border in "a typical day in the wettest and coldest October in memory",[21] fell seriously ill at the Favorita Palace, Vienna, and he died on 20 October 1740 in the Hofburg.[22] In his Memoirs Voltaire[23] wrote that Charles' death was caused by consuming a meal of death cap mushrooms.[24] Charles' life opus, the Pragmatic Sanction, was ultimately in vain. Maria Theresa was forced to resort to arms to defend her inheritance from the coalition of Prussia, Bavaria, France, Spain, Saxony and Poland—all party to the sanction—who assaulted the Austrian frontier weeks after her father's death. During the ensuing War of the Austrian Succession, Maria Theresa saved her crown and most of her territory but lost the mineral-rich Duchy of Silesia to Prussia and the Duchy of Parma to Spain.[25]

Emperor Charles VI has been the main motif of many collectors' coins and medals. One of the most recent samples is high value collectors' coin the Austrian Göttweig Abbey commemorative coin, minted on 11 October 2006. His portrait can be seen in the foreground of the reverse of the coin.[26]

^Edward Crankshaw: Maria Theresa, A&C Black, 2011. And also: «[...] after a day of hunting, the emperor fell ill with a cold and fever. Upon his return to his hunting lodge, Charles requested his cook to prepare him his favorite dish of mushrooms. Soon after eating them, he fell violently ill. His physicians bled him but to no avail» (Julia P. Gelardi: In Triumph's Wake: Royal Mothers, Tragic Daughters, and the Price They Paid for Glory, Macmillan, 2009).

^In the first days of October 1740, in a cold day of pouring rain Emperor Charles VI, «in spite of the warnings of his physicians» (Eliakim Littell, Robert S. Littell: Littell's Living Age, Volume 183, T.H. Carter & Company, 1889, pg. 69), went to hunting ducks on the shores of Lake Neusiedl, close to the Hungarian border and he had come back chilled and soaked through to his little country palace at La Favorita; on his return, though he was feverish and suffering from colic, the Emperor persisted in eating one of his favourite dishes, a Catalan mushroom stew («a large dish of fried mushrooms» for the Littell brothers), prepared by his cook. He spent the night between 10 and 11 October vomiting. The following morning he was gravely ill, brought down by a high fever. Carried slowly to Vienna in a padded carriage, he died in the Hofburg nine days after.

^«Charles the Sixth died, in the month of October 1740, of an indigestion, occasioned by eating champignons, which brought on an apoplexy, and this plate of champignons changed the destiny of Europe» (Voltaire: Memoirs of the Life of Voltaire, 1784; pp. 48–49).

^Wasson RG. (1972). The death of Claudius, or mushrooms for murderers. Botanical Museum Leaflets, Harvard University23(3):101–128.

^Browning, Reed: The War of the Austrian Succession, Palgrave Macmillan, 1995, ISBN0-312-12561-5, 362.